THE CHEMICAL COMPOSITION OF THE
EARTH'S ORIGINAL ATMOSPHERE

John C. WaltonLecturer in Chemistry
University of St. Andrews, Fife, Scotland

It has often been postulated that the original atmosphere of the earth was a
reducing one. Such an atmosphere, unlike the present oxidizing kind, would facilitate the
formation and survival of those molecules believed to be associated with the spontaneous
origin of life. The author discusses some of the cosmological, geological, and biological
evidence related to this important issue.

According to a popular theory of the origin of life, which has
attained almost the status of orthodoxy in the world of science, a variety of
physico-chemical processes led to the formation and accumulation of biologically
important, simple organic compounds early in the earth's history. These molecules combined
together to give more and more complex products until one was formed that could be called
living (1). Oxygen rapidly degrades the majority of organic compounds and inhibits many of
the metabolic reactions in living systems, and therefore the original proponents of this
theory postulated that the primitive earth must have had a reducing atmosphere devoid of
oxygen (2, 3).
In 1953 Miller carried out his famous experiments in which reducing
atmospheres of methane, ammonia, water and hydrogen were subjected to electric discharges
and a wide variety of organic compounds, including amino acids, were formed (4). Numerous
experiments purporting to simulate primitive earth conditions with a variety of gas
mixtures and energy sources have subsequently confirmed that amino acids and other
bio-organic molecules are synthesized under reducing conditions (5, 6), but not in the
presence of oxygen (6-9). Bio-organic compounds are not forming in significant quantities
in the earth's present atmosphere or hydrosphere (except in living organisms) and
therefore the initial steps in this mechanism for the origin of life depend crucially on
the nature and composition of the earth's original atmosphere. In this article the
cosmological, thermodynamic, and geological evidence bearing on this question is reviewed.
It is found that the facts are in better agreement with an early oxidising atmosphere
similar to the present one.

A. FEASIBILITY AND STABILITY OF A REDUCING ATMOSPHERE

1. Cosmological Evidence

In the most generally held theory of the origin of the solar system,
the sun and planets are supposed to have condensed from a dust cloud having the same
average composition as the universe. The cosmic abundances of a selection of elements are
given in Table 1, which shows that hydrogen and helium are by far the most abundant.

TABLE 1. Terrestrial and cosmic abundances of selected elements.

ELEMENT

ATOMIC WEIGHT

COSMIC ABUNDANCE (Si=106)

TERRESTRIAL ABUNDANCE (ppm)

TERRESTRIAL ABUNDANCE
COSMIC ABUNDANCE

Oxygen

16

2.9×107

456,000

Silicon

28

1 ×106

273,000

Aluminium

27

9 ×104

83,600

Iron

56

8.5×104

62,200

Calcium

40

7.3×104

44,600

Magnesium

24

1 ×106

27,640

Sodium

23

4.2×104

22,700

Hydrogen

1

3.2×1010

1,500

Carbon

12

1.7×107

180

Nitrogen

14

3 ×106

19

Helium

4

2.6×109



Neon

20

10-10

Argon

36

10-9

Krypton

84

10-7

Xenon

131

10-7

Sources: see ref. (10)

Early workers in this field therefore concluded that the earth must
originally have had an atmosphere rich in hydrogen and the reduced compounds of other
elements such as ammonia, methane and water, somewhat similar to the atmosphere known to
exist on the heavy planets such as Jupiter and Saturn, but containing less helium and
hydrogen as these light elements would escape from the earth's gravitational field. A
major difficulty with this view is the relatively low amount of the rare gases on the
earth compared to their cosmic abundances. The ratios of the cosmic abundances of the rare
gases to their terrestrial abundances are given in Table 1, and it is apparent that
the earth contains many orders of magnitude less neon, argon 36, krypton and xenon than it
should if it were condensed from average cosmic matter.
This view has now largely been abandoned in favour of the idea that
volatile elements such as hydrogen were lost from the earth during the condensation stage
while the gravitational field was weak. The earth's reducing atmosphere is then supposed
to have accumulated by outgassing of the interior by volcanic and allied activity. It is
difficult to accept that xenon and krypton with atomic weights of 131 and 84 respectively
could be so thoroughly lost from this early gravitational field, and yet methane, ammonia
and water, and a host of lighter compounds be retained. Even if the additional hypothesis
that these molecules were retained in chemically combined form, such as carbonates and
ammonium salts, is advanced, it would still be expected that much of the rare gases should
be retained by occlusion within dust particles and larger fragments composing the cloud.
Argon, for example, is trapped with fair efficiency in rocks, and this is an important
factor in the potassium-argon dating technique. The extremely low terrestrial abundances
of the rare gases are still, to say the least, surprising. The chemically combined forms
of carbon and nitrogen so far detected in cosmic dust clouds are volatile, low molecular
weight compounds such as formaldehyde, hydrogen cyanide, methanol and acetaldehyde (11).
The most natural conclusion from this is that the earth's composition has always been
unlike the average of the universe.
Gases evolving from present-day volcanoes vary in composition with
locality and time. Typical compositions of the emanations from some volcanoes and
fumaroles are shown in Table 2. Steam and carbon dioxide are usually the main
constituents, but occasionally hydrogen, carbon monoxide, sulphur dioxide or hydrogen
chloride may be important. Methane and ammonia are rarely found, and then usually in trace
quantities only. The hypothesis that the earth acquired a reducing atmosphere by volcanic
outgassing finds little support in the evidence from present-day volcanoes. The hypothesis
can be bolstered by making the additional assumption that volcanic gases were much more
reducing in the past (13). This sheds no further light on the problem since it is merely
the complement of the original reducing atmosphere hypothesis. It is interesting to note
that Precambrian igneous rocks closely resemble their modern counterparts in chemical
composition (see below).

TABLE 2. Composition of volcanic and fumarolic gases (12).

VOLCANO OR FUMAROLE

CO2

CO

CH4

NH3

H2

HCl

HF

H2S

SO2

%
H2O

Showa-shinzan
A1 Japan

65



0.1

0.1

25

5

3

0.1

1.7

99.2

Showa-shinzan
B4b Japan

92



0.2



4

2

0.5

1.0

0.4

90.1

Sheveluch
Kamchatka

60

3





5

1





32



Biliuka
Kamchatka



28





26

46









Valley of 10,000
Smokes, Alaska

60



0.2









40



99.9

Mount Hood
Oregon

96



0.1



0.3





3



98.7

White Island
New Zealand

73









25





2

95

Kilauea
Hawaii

96

1.7

0.4











1.6



Novarupta
Alaska

79

0.2

12.5









7.9



98.8

Mont Pelée
Martinique

100



















Ambrym
New Hebrides

88

1.2













9.8

22.7

Nyiragongo
Zaire

84

5.6





1.6







9.1

43.2

Vesuvius
Italy

84







9.5

6.0







67.7

Soufrière de la
Guadeloupe

69



1.3









6.1

24

91

Composition given as volume percent for each gas excluding water.
Water is given as percentage of total gases.

2. Kinetic and Thermodynamic Considerations

Can molecules such as methane, ammonia, or carbon monoxide in
contact with the ocean, the earth's crust, and in the presence of sunlight persist for
geologically significant periods of time? Methane is the thermodynamically stable carbon
compound in the presence of excess hydrogen:

CO2 + 4H2

® CH4 + 2H2O (liq)

K = 8×1022

CO + 3H2

® CH4 + H2O (liq)

K = 2.5×1026

C + 2H2

® CH4

K = 8×108

In the absence of hydrogen, however, ultraviolet light and electric discharges readily
decompose methane, which also reacts with water to give carbon dioxide and carbon
monoxide. Carbon monoxide reacts quite rapidly with ocean water (pH 8.1) to give formate:

CO + OH¯ ® HCO2¯

and it is unlikely that carbon monoxide could persist even in a reducing atmosphere.
Ammonia is formed from nitrogen and hydrogen in a fairly favourable
process:

1/2N2 + 3/2H2
® NH3
K = 8×102

but it is also rapidly destroyed by ultraviolet radiation; so that a quantity of
ammonia equivalent to the present atmospheric nitrogen would be degraded in this way in
30,000 years (14). Ammonia is also highly soluble in water and would dissolve in the ocean
to form ammonium ions thus effectively being removed from the atmosphere (15).
The indications are that carbon monoxide and ammonia could not persist
in the atmosphere in contact with the ocean, and the availability of methane depends on a
constantly supplied reservoir of hydrogen in the atmosphere. At present there is
relatively little hydrogen available in the earth's crust (see Table 1). According to
the dust cloud model for the formation of the earth, the majority of the hydrogen and
other volatiles would have been lost before consolidation of the earth (16). Any hydrogen
surviving this process, or being produced by photodissociation of water vapour or by
volcanic emanation, is also rapidly lost from the earth's gravitational field because of
its low molecular weight.
The exact rate of loss of hydrogen is difficult to assess, since it
depends on the rate of diffusion of hydrogen to the escape layer, and the temperature and
diameter of the exosphere, none of which can be known for the primitive earth. At present
the temperature of the escape layer is extremely high (~1500ºK) so that almost all
hydrogen getting there escapes. It seems doubtful that hydrogen (and hence methane,
ammonia or other reduced gases) could ever have reached substantial concentrations in the
atmosphere, and in view of this the formation and persistence of a reducing atmosphere
must be considered unlikely under any geologically plausible conditions.
One of the most significant facts in this area, which is often
overlooked, is the great abundance of oxygen available on the earth. It is by far the most
abundant element in the earth's crust (see Table 1) which is largely composed of
oxides of silicon, calcium, aluminium and other metals (17). Since oxygen is also very
abundant in the universe, being third in magnitude after hydrogen and helium, it is
probable that very great amounts must have been present from the beginning, regardless of
the mechanism for the formation of the earth. Furthermore, oxygen is also overwhelmingly
abundant combined with hydrogen in the hydrosphere.
If we assume for the moment that oxygen had been absent from the
atmosphere at some time, then several mechanisms exist which would rapidly build up its
concentration. Water vapour is dissociated by ultraviolet radiation in the upper
atmosphere to give hydrogen, most of which escapes into space, and oxygen which is
retained by the earth's gravitational field:

2H2O ® 2H2 + O2

Urey (18) has suggested that production of oxygen in this way would be limited because
the oxygen formed would absorb the ultraviolet light needed to photodissociate the water.
This self-regulating mechanism depends on the difference in vertical distribution of
oxygen and water in the atmosphere. Oxygen is distributed exponentially, but water is
arrested in a cold trap in the troposphere. Oxygen would rise much higher in the
atmosphere, and since oxygen and water absorb radiation substantially in the same region
of the spectrum, the water would be screened from further dissociation as soon as
sufficient oxygen built up (19). Berkner and Marshall (20) presented detailed calculations
which showed that this mechanism would limit oxygen in the atmosphere to about one
thousandth of its present atmospheric level. Brinkmann, however, reconsidered this problem
and showed that Berkner and Marshall's calculations were invalid because they ignored the
pathlength dependence of the oxygen absorption and the dissociation of water when it was a
minor absorber (21). Brinkmann showed that the Urey self-regulation mechanism is much less
effective than was formerly supposed, and that oxygen could have reached an appreciable
fraction of the present atmospheric level from water dissociation alone. He concludes that
the atmosphere could have been oxidising over a large fraction of geologic time. Direct
observations from the moon during the Apollo 16 mission revealed that substantial amounts
of hydrogen are leaving the earth's atmosphere, due to photochemical water dissociation in
the upper atmosphere (73).
A second important mechanism for the production of oxygen is
photosynthesis by living organisms. The rate of oxygen production is extremely high, and
it is estimated, for example (22), that at the present time all the oxygen in the
atmosphere passes through the photosynthetic cycle in 2000 years, and all the carbon
dioxide in 300 years. The oldest known rocks contain evidence of photosynthetic organisms,
although this is by no means conclusive (see below). Van Valen has pointed out that when
oxygen is produced in photosynthesis, a stoichiometrically equivalent quantity of reduced
carbon is also formed (22). If oxygen had built up from a very low concentration to the
present atmospheric level, there should be an accumulation of reduced carbon in the earth
equivalent to far more than the reserves of coal and oil now known. Van Valen considers
that there is no satisfactory mechanism for regulating oxygen concentrations at very low
levels.
When sedimentary rocks are subjected to high pressures and temperatures
in metamorphic processes in the earth's crust, the dominant chemical process occurring is
progressive loss of water:

With carbonate minerals carbon dioxide can be formed, and oxygen can also be produced
in gas-solid reactions involving abundant minerals:

CaCO3 + SiO2 ®
CaSiO3 + CO2
calcite
wollastonite

Fe2O3 + SiO2 ® Fe2SiO4 + ½O2
hematite
fayalite

This process is probably not important as a direct contributor to atmospheric oxygen,
since volcanic gases rarely contain oxygen, but the dominant position of water and carbon
dioxide in volcanic emanations is readily explained. A facile route is seen to exist for
the transfer of oxygen from the vast reservoirs in the rocks to the ocean where it can be
used in photolysis or photosynthesis and thence to the atmosphere.

B. GEOLOGICAL EVIDENCE

It might be expected that conclusive evidence regarding the earth's
original atmosphere could be obtained from a study of the oldest known rocks in the
Precambrian formations. Unfortunately, Precambrian rocks have suffered extensively from
metamorphic and diagenetic processes which have altered their original form and content
chemically and physically. Most of the exposed rocks have suffered so much that most of
the evidence of their early history has been destroyed. An additional problem is the
contamination of the ancient rocks with material of recent origin carried in by
circulating ground water or processes of diffusion. Interpretation of the evidence is a
highly subjective process and, not surprisingly, geologists are divided amongst several
schools of thought. Rutten has presented the position favouring an early reducing
atmosphere very persuasively in his book The Origin of Life by Natural Causes
(23). Another school of thought favours the view that the atmosphere has been oxidising
since the formation of the earliest crustal rocks (24).

1. Oxidation State of Iron in Precambrian Rocks

Evidence commonly cited as favouring an early reducing atmosphere is
the preponderance of the reduced form of iron in Precambrian formations. Rankama, for
example, reported that ferrous oxide predominated over ferric oxide in Precambrian
conglomerates from Suodenniemi, Finland (25). Nanz also found a preponderance of ferrous
iron in Precambrian slates from Michigan and Wisconsin (26). The fact that oxidised forms
of iron occur at all in the Precambrian might be taken to indicate an oxidising
atmosphere. Even the oldest known sedimentary rocks from the Isua iron formation in
Greenland contain bands of magnetite (Fe3O4) of intermediate
oxidation state (27). Vast quantities of magnetite and hematite are present in Precambrian
iron formations, which would require an immense amount of oxygen for their formation from
reduced iron compounds. Where could all this oxygen come from if not directly or
indirectly from the atmosphere (28)?
The preponderance of ferrous iron might seem significant until it is
recognised that metamorphic processes often lead to partial reduction of iron (26) and
that many reducing environments are known on the earth at the present day. Reducing
conditions prevail deep in the earth's crust, and materials brought to the surface by
volcanic activity or rising hydrothermal waters are commonly in reduced form. Fyfe
suggested that the ferrous oxide in Precambrian rocks is simply a reflection of the
enhanced volcanism prevailing in earlier times (24). Reducing conditions also hold sway in
deep waters such as the Norwegian fjords and the Black Sea (29) and in the northern Indian
Ocean (30). Throughout geologic time and even today, ferrous iron is effectively
transported and deposited by subsurface waters (31). The fact that all oxidation states of
iron, from hematite to magnetite to siderite (FeCO3) to pyrite (FeS2),
have been found in sediments of all ages merely indicates that the oxidation state of
sediments depends primarily on local conditions that do not reflect the average
atmospheric composition at the time of deposition.

2. Uraninite and Pyrite in Precambrian Gold-Uranium Reefs

Precambrian deposits in the Dominion and Witwatersrand Reefs, South
Africa; Serra de Jacobina, Brasil; and Blind River, Ontario, districts contain ancient
conglomerates and sands cemented to a very hard rock carrying grains of uraninite (UO2),
pyrite and ilmenite (FeTiO3). The origin of these gold-uranium blanket ores has
been controversial for more than half a century. In 1958 Ramdohr suggested they could be
considered as sediments laid down under an anoxygenic atmosphere and presented evidence
for repeated cycles of weathering-erosion-transportation-sedimentation, indicating that
these beds must have been in repeated contact with the contemporaneous atmosphere. The
reduced state of the pyrite and uraninite (the stable form of uranium under present-day
atmospheric conditions is UO3) was taken as an indication of an anoxygenic
atmosphere at the time of deposition (32, 23). Davidson, on the other hand, maintains that
the uranium and pyrite ore bodies have been formed within the earth's crust long after the
mother rock sediments were laid down, by infiltration of hydrothermal metal-bearing
solutions, and that they cannot be explained in terms of abnormal atmospheric conditions
(24, 28). Even Rutten, an enthusiastic supporter of the reducing atmosphere hypothesis,
admits that the deposits show extensive effects of hydrothermal processes (33).
If, however, it could finally be proved that the uraninite and pyrite
deposits are sedimentary in origin, there is no need to invoke an anoxygenic atmosphere to
explain their formation. The thermodynamically stable uranium oxide under an atmosphere of
oxygen is UO3, and consequently UO2 converts to this, but the rate
of conversion depends strongly on the physical form of the uraninite. Conversion is rapid
for finely divided or powdered UO2, but granular or compacted material is
stable indefinitely (34) and it is even used, fabricated into rods, as fuel elements for
nuclear reactors. Grains of uraninite would be expected to remain unchanged during
erosion, transportation and deposition even under the present oxygenic atmosphere. Zeschke
has shown that uraninite is transported as minute grains by the Indus River in Pakistan at
the present day (35). Furthermore, in the Mozaan rocks of Swaziland, deposited
concurrently with the Witwatersrand strata, and in the Lorrain sandstones of Ontario,
almost contemporary with the Blind River conglomerates, heavy mineral assemblages,
completely of modern aspect, are widely developed (24). This indicates that the
uranium-pyrite ores, which are themselves strikingly similar to recent deposits except for
the presence of uraninite and pyrite grains (36), cannot be explained in terms of abnormal
atmospheric conditions.
Many of the Precambrian mineral deposits are associated with
carbonaceous matter. Uraninite and pyrite are frequently found in association with
thucolite, a carbonaceous mineral, and kerogen, which is graphitized organic matter
resembling medium- to high-grade coal (37, 38). Pyrite is deposited in present-day
environments, such as the Black Sea, by the action of bacterial sulphate-reducers (39).
Micro-organisms are known to play an important role in the deposition and concentration of
many minerals. The presence of carbonaceous matter in Precambrian ore deposits has
generated interest in the possibility that they owe their formation to oxygen-producing or
oxygen-utilising micro-organisms (40). According to Koen the uranium was fixed and
concentrated by biological action (41), and it has been observed that the gold-uranium
reefs of the carbon seam type of the Kaapvall Craton are confined to environments expected
to favour algal growth (42). Schidlowski, on the other hand, has advocated an origin for
the thucolite involving migration of biogenic compounds into the conglomerates where they
are exposed to radiation from the uranium and undergo condensation and polymerization
reactions, finally solidifying as carbonaceous material around individual uraninite grains
(37). Doubt has been cast on this hypothesis by observations on Precambrian thucolite
occurrences in Australia which showed that the substance does not have the properties
expected of a radiolytic polymer (43).
Obviously no final conclusions can be drawn at present, but there must
be a strong presumption that photosynthetic organisms are involved, particularly for
pyrite. Further support for this view comes from the isolation of chemical fossils
including porphyrins from the carbonaceous material (see below).

3. Precambrian Banded Iron Formations

Another type of rock which is supposed to provide evidence of an
anoxygenic atmosphere is the banded iron formation; composed of thin laminae of finely
crystalline silica alternating with thin laminae of iron ore, the iron being mainly in the
ferrous state. Cloud has repeatedly championed the view that the banded iron formations,
which are most common in the middle Precambrian, represent unoxidised sediments laid down
under a reducing atmosphere and that as the atmosphere became more oxidising towards the
end of the middle Precambrian, the banded iron formations cease and are replaced by
oxidised sediments known as red beds (44, 23).
There are many difficulties associated with this view. Even the most
ancient banded iron formations contain hematite and magnetite (27), which were at least
partly laid down in the primary deposition of the formations (45). Fully oxidised minerals
also occur in formations contemporary with the banded iron formations. Bedded iron ores in
the Pretoria series of the Transvaal system (middle Precambrian) are mainly
hematite-chamosite or magnetite-chamosite oölites akin to the Wabana ores of Newfoundland
and other Phanerozoic formations (28), which clearly contradicts the hypothesis of an
anoxygenic atmosphere. There is no clear distinction in time between the cessation of
banded iron formations and the onset of red bed formation. Banded iron formations of late
Precambrian and even younger age are known, although they are not common (45, 31). A
minority of geologists also consider that the banded iron formations are of volcanic
origin (46), their reduced state having no direct connection with atmospheric conditions.
The majority view is that the deposits are sedimentary in origin, but
the orthodox explanation of the thin laminations of iron ore does not require the presence
of an anoxygenic atmosphere. It is proposed that weathering processes bring the more
soluble ferrous iron down to deep lakes where it is trapped in solution in the deeper
oxygen-free waters. Ferrous iron is then precipitated as soon as mixing of the waters
brings oxygen to the lower water body (47, 48). Environments such as this are common in
present-day lakes.
Red beds are sandstone, siltstone or mudstone made of detrital grains
set in a reddish-brown matrix or cemented by precipitated ferric oxide. They occur in
great variety and complexity, and their origin and mode of formation have been something
of an enigma for years (49). Red beds are in fact known from the middle and early
Precambrian, contemporary with the banded iron formations, although they are not common.
Red beds are found in the Uyansk and Uchursk series of the Siberian platform, in the
Roraima formation, South America, in the Lorrain formation of the Huronian in Ontario and
possibly also in the much older Muruwa formation of Guiana (28).
Most of the oxidised iron in red beds is not considered to be an
original constituent of the rock but to have been produced in situ by
post-positional chemical changes (47, 48). There is some evidence that the relative lack
of red beds in the early Precambrian is because corresponding deposits are in the process
of metamorphism whereby the red pigment is dissolved out by ground waters containing
magnesium, and the mineral chlorite is then precipitated (28).

4. Geological Evidence of an Early Oxygenic Atmosphere

The proponents of an early reducing atmosphere have focused
attention on certain special features of Precambrian strata considered above, but this
evidence needs to be evaluated in the light of the overall situation in the Precambrian.
The oldest rocks on earth are rather similar to modern ones, the sedimentary rocks being
"rather normal," indicating that the oceans at the time were rather similar to
modern ones (50). Much the same kinds of sedimentary rocks are found in Precambrian as in
Paleozoic and younger rocks (51, 52): they are water laid and imply the existence of a
substantial hydrosphere and atmosphere for weathering processes (53). Sedimentary rocks
found in the Precambrian are lithologically comparable with those of younger age (54, 55),
and igneous rocks of early Precambrian age closely resemble modern rocks in chemical
composition (46). Although the size of the ocean in earlier times remains a matter of
debate, there is no evidence that sea water chemistry has ever changed substantially, and
it is commonly assumed that its pH and composition have been constant from the earliest
times (50, 56). The atmosphere-hydrosphere-lithosphere interaction is an extremely complex
one, and the similarities in ancient and modern rocks and sediments clearly imply an
atmosphere little different from the present.
All minerals except oxides are unstable in chemical weathering under an
oxygenic atmosphere. The ions derived from weathering when transported into areas of
sedimentation recombine, mostly forming clay minerals. Clays are as abundant in the early
Precambrian as at present (57). In a reducing atmosphere, weathering is expected to be
much slower and quite different in character, since feldspars, dark minerals, sulphides
etc., would be chemically unaffected, and weathering would take place by minor physical
processes such as frost splitting and sun blasting (58). The maturity of many Precambrian
sediments, the roundness of sand grains, and the well-worn pebbles and cobbles found in
early Precambrian conglomerates demonstrate that weathering, erosion and sedimentation
went on then as now. Pettijohn considers that such mature weathering could not have been
accomplished in the absence of some kind of plant cover (72). Modern lateritic soils
require humic acid derived from vegetation, living in an oxygenic environment, for their
development. Precambrian banded iron formations, which resemble lateritic soils, are
thought by some to have been formed in a similar way. This view is supported by the
finding of frequent intercalations of graphite, possibly of biological origin in the iron
formations (46).
Limestone is produced at present by the growth of green algae in
calcium-bicarbonate-rich ground waters in an oxygenic environment. Inorganic deposition of
limestone in stalactite caves is also known, but this is normally almost negligible in
sunlit areas. Limestone deposits, although not abundant, are found in early, middle and
late Precambrian times. Some of these deposits may be biogenic; for example, those
described from the Bulawayan system, of early Precambrian age, by MacGregor (59, 60) and
the prolific development of similar structures in the middle Precambrian of Finland (61)
which are entirely comparable with recent algal reefs.
Since limestone is known to be deposited by green algae living in an
aerobic environment, one may propose that the Precambrian atmosphere, at the time of
deposition of the limestone reefs, was also oxidising. Of course it is always possible to
postulate that the metabolism of lime-depositing algae in the past must have been quite
different (57). The soundest procedure, however, is to utilise the modern analogy and work
from the known present to the unknown past. Limestone deposition by anaerobic fermenting
organisms, though possible, would be expected to produce material of quite different
chemical and physical composition, whereas the Precambrian deposits are notably similar to
modern equivalents.
Chert or flint is an extremely fine-grained silica which forms, at the
present day, by the chemical action of sea water on the siliceous skeletons of plankton
such as Diatoms and Radiolaria. It also forms by concretionary action in rocks after their
emergence from the sea, and in lacustrine limestones. Enormous amounts of cherts occur in
Precambrian strata such as the banded iron formations, where indirect evidence for a
biological origin is provided by its association with organic matter, including in some
cases actual microfossils (38).
Phosphorites are deposits which in modern examples of formation develop
from phosphatic phytoplankton living by photosynthesis in the uppermost 50 m of the
ocean. Many occurrences of Precambrian phosphorites are known, such as those from the
Upper Krivoi Rog series of middle Precambrian age, or the deposits of Slyudganka in
eastern Sayan and the even older phosphorites in the Baikal region, all from the U.S.S.R.
(28). The present-day mode of formation of cherts and phosphorites strongly suggests the
Precambrian equivalents developed in an oxygenic atmosphere.

5. Macrofossils, Microfossils and Chemical Fossils

Considerable evidence has accumulated in recent years indicating
that the carbonaceous matter found in the Precambrian strata is of biosynthetic origin.
The nature of this material also suggests that some of it was derived from
oxygen-utilising and oxygen-producing organisms. Great caution needs to be exercised in
the evaluation of this data because of the problem of contamination. Recent studies with
the finest grained cherts and shales from the Precambrian have shown that they are
significantly permeable to the flow of fluids and that organic material of modern
derivation can be carried into the rock matrix by circulating ground waters (62, 63). At
present there are no wholly sound criteria for distinguishing between organic material
laid down with the host rock and contamination of recent origin.
A second problem concerns the genuineness of the microfossils, many of
which have very simple forms such as spheres or rods. Inorganic artifacts such as
colloidal salts after drying can give rise to similar structures, and artifacts composed
of iron, silica and other materials could certainly occur in sediments. The microfossils
are generally accepted as genuine, but the possibility of "pseudofossils" cannot
be ruled out.
The only macrofossils that are of widespread occurrence in the
Precambrian are stromatolites: metric-sized mounds with a characteristic laminated
structure. These are reef-like remnants usually presumed to have been formed from
precipitated mineral matter on the enlarging surfaces of microbial communities (usually
dominated by blue-green algae) growing by photosynthesis (64). Stromatolites are known
from the Bulawayan group of early Precambrian age, and are widespread in the middle and
late Precambrian. The fossil stromatolites bear a marked resemblance to modern ones
forming at Hamelin Pool near Shark Bay, Australia (64, 65).
The Onverwacht and Fig Tree groups of the Swaziland system, South
Africa, are considered to be the oldest relatively unmetamorphosed rocks known. Numerous
globular, filamentous and rod-like microfossils have been found in cherts and shales from
both these rock groups. Analysis of these structures has shown that the outer walls
definitely contained carbon compounds and many authorities accept that they are of
biological origin; but because of their very simple structure some doubts remain (38, 64,
67). Undoubted microfossils of diverse and structurally complex microorganisms are
relatively abundant in rocks of middle Precambrian age. For example a great wealth of
forms is encountered in the Gunflint formation (66), with variations between filamental
and spheroidal, between septate and non-septate, all pointing to a varied and intricate
morphology, such as is only exhibited by living matter. Other middle and late Precambrian
localities where microfossils have been discovered are indicated in Table 3 (38, 67).

Part of the carbonaceous matter in ancient sediments is extractable
by solvents, but most of it is graphitized material (kerogen). A variety of chemical
fossils, or "biological markers," have been identified in the extractable
fractions from Precambrian cherts and shales. The occurrence of n-alkanes,
isoprenoids such as pristane and phytane, steranes, fatty acids, metal chelated
porphyrins, amino acids and carbohydrates from the early, middle and late Precambrian
formations is indicative of a biological origin for much of the material (68). The
porphyrins, and pristane and phytane, which are possibly degradation products of the
side-chains of chlorophylls, are particularly suggestive of photosynthetic organisms. They
have been identified in the most ancient sediments from the Onverwacht and Fig Tree series
(38). Amino acids have also been found in rocks throughout the Precambrian (69); however,
serine and threonine, which are known to degrade rapidly on the geological time scale,
were also discovered. The amino acids in the Fig Tree and Gunflint formations were also
shown to comprise the L-isomers only (70), whereas racemisation is known to be rapid (68).
Contamination from modern sources seems the probable explanation, and the significance of
all the chemical fossils is called into question.
The non-extractable kerogen was almost certainly laid down at the time
of formation of the host rock. Fractionation of the stable carbon isotopes mass 12 and 13
occurs in living systems and is measured in terms of d13C,
negative values indicating material of biological origin and near zero or positive values
characterising inorganic carbon-containing remains (71). In Table 3 are shown the
Precambrian formations having negative d13C values
characteristic of biogenic material.
Taken together this comprises an impressive array of evidence that life
has been present on the earth since the formation of early Precambrian sediments.

C. CONCLUSIONS

The occurrence of a reducing atmosphere on the earth depends on the
availability of hydrogen, but hydrogen is known to escape rapidly from the earth's
gravitational field because of its low molecular weight. Hydrogen is not a major
constituent of the crust at present, and according to the dust cloud theory of the earth's
formation, no large reservoir of hydrogen survived consolidation of the earth.
Furthermore, there do not appear to be efficient processes for maintaining a flow of
hydrogen to the atmosphere, volcanic evolution of hydrogen being minor. The origin and
persistence of a reducing atmosphere is therefore difficult to envisage, particularly when
the reactivities of reduced gases such as carbon monoxide and ammonia with the ocean are
considered.
Oxygen must always have been of major importance in the earth's crust;
the composition of the earliest igneous and sedimentary rocks and of the Precambrian
ocean, as well as its cosmic and terrestrial abundance, all attest this. Efficient
gas-solid reactions are known which transfer oxygen in the form of water, from the vast
reservoirs in rocks to the ocean, and two major processes, photolysis of water vapour and
photosynthesis, transfer oxygen to the atmosphere. In the light of this evidence, no other
conclusion than that oxygen has always been an important constituent of the atmosphere
seems possible.
Geological evidence indicates that rocks from the earliest Precambrian
are lithologically quite normal and have similar modern counterparts. Weathering,
transportation and sedimentation appear to have taken place essentially as at present.
Certain sediments containing minerals in reduced form can readily be accounted for in
terms of local reducing conditions, such as are found in many areas today, or they are
found to be stable in their lower oxidation state sufficiently long enough for erosion,
transportation and deposition. There appears to be no persuasive evidence that the
atmosphere has ever differed substantially from its present composition. The presence of
oxygen in the earth's original atmosphere would, of course, have a dramatic inhibitory
effect on the synthesis and accumulation of organic molecules and would virtually rule out
the possibility that life arose in this way.
The presence of limestone, and other probably biogenic sediments, of
stromatolites, microfossils, chemical fossils and biogenic kerogen in early Precambrian
rocks suggests that life originated virtually simultaneously with the formation of the
crust of the earth.

ACKNOWLEDGMENTS

The author thanks Professor Lord Tedder, Dr. C. Mitchell and Mr. W.G.C. Walton for
reading the typescript and making valuable comments and suggestions.

FOOTNOTES

S.L. Miller and L.E. Orgel. 1974. The origins of life on the earth.
Prentice-Hall, Englewood Cliffs, New Jersey, p. 1.

S.W. Fox and K. Dose. 1972. Molecular evolution and the origin of life. W.H.
Freeman, San Francisco, chapter 4.

P.H. Abelson. 1956. Amino acids formed in "primitive atmospheres."
Science 124:935; 1957. Discussion of S.L. Miller's The formation of organic compounds on
the primitive earth. New York Academy of Sciences Annals 69:274; 1966. Chemical events on
the primitive earth. Proceedings of the National Academy of Sciences 55:1365.

It should be recognised that if this mechanism is efficient, ultraviolet light
would also not be available for the synthesis of amino acids etc., except in the upper
atmosphere above the oxygen and ozone screen, where they would be particularly vulnerable
to decomposition.